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A RAID-1 set consists of several disk drives which are exact copies
of each other. In the event of a mirror failure, the RAID driver
will continue to use the operational mirrors in the set, providing
an error free MD (multiple device) to the higher levels of the
kernel. In a set with N drives, the available space is the capacity
of a single drive, and the set protects against a failure of (N - 1)
drives.

Information about Software RAID on Linux is contained in the
Software-RAID mini-HOWTO, available from
http://www.tldp.org/docs.html#howto. There you will also
learn where to get the supporting user space utilities raidtools.

If you want to use such a RAID-1 set, say Y. To compile this code
as a module, choose M here: the module will be called raid1.